Fire-resistant steel material superior in weld heat affected zone reheat embrittlement resistance and low temperature toughness and method of production of same

ABSTRACT

The present invention provides a fire-resistant steel material superior in weld heat affected zone reheat embrittlement resistance and low temperature toughness when welded by large heat input and exposed to fire and a method of production of the same, that is, a material containing C: 0.012 to 0.050%, Mn: 0.80 to 2.00%, Cr: 0.80 to 1.90%, and Nb: 0.01 to less than 0.05%, restricting Cu to 0.10% or less, containing suitable quantities of Si, N, Ti, and Al, restricting the contents of Mo, B, P, S, and O, and having a balance of Fe and unavoidable impurities, having contents of C, Mn, Cr, Nb, and Cu satisfying −1200C−20Mn+30Cr−330Nb−120Cu≧−80, having a steel structure as observed by an optical microscope of an area fraction of 80% or more of a ferrite phase, and having a balance of the steel structure of a bainite phase, martensite phase, and mixed martensite-austenite structure.

TECHNICAL FIELD

The present invention relates to a fire-resistant steel material superior in weld heat affected zone reheat embrittlement resistance and low temperature toughness and a method of production of the same.

BACKGROUND ART

Steel structures of buildings etc. are required to exhibit strength for a certain period to prevent collapse and enable residents to escape when exposed to fire. However, in general, steel materials fall in strength when exposed to high temperatures. Therefore, in the past, as a countermeasure, the technique has been employed of providing the steel material with a fire-resistant covering for the purpose of keeping down the rise of temperature of the steel material at the time of fire.

On the other hand, in recent years, due to environmental issues and issues of aesthetics etc., art has been proposed for forming steel structures without using fire-resistant coverings. Various scales of fires and ambient temperatures etc. may be envisioned, so when not providing the steel materials with fire-resistant coverings, the steel materials supporting the strength of the structures are required to be made as high as possible in strength at high temperatures. The property of not easily falling in strength even at high temperatures is called the “fire-resistant performance”.

For steel materials providing such fire-resistant performance, in the past Mo has been positively utilized. Mo is an element useful for raising the high temperature strength by precipitation strengthening. However, in recent years, the price of Mo has skyrocketed, so art based on alloy designs not relying on addition of Mo has been proposed (for example, see PLTs 1 to 4).

Further, when a steel structure is exposed to fire, the heat affected zone (below, sometimes called the “HAZ”) of the weld joint sometimes cannot keep up with the deformation and breaks. The low level of deformation ability of the HAZ when exposed to a high temperature (below, sometimes called “HAZ reheat embrittlement”) is particularly remarkable in steel containing Mo or B. For this reason, steel has been proposed using solution strengthening by Nb to raise the high temperature strength and suppress the addition of Mo and B (for example, see PLT 5).

Citation List

Patent Literature

PLT 1: Japanese Patent Publication (A) No. 2002-115022

PLT 2: Japanese Patent Publication (A) No. 2007-211278

PLT 3: Japanese Patent Publication (A) No. 2007-224415

PLT 4: Japanese Patent Publication (A) No. 2008-88547

PLT 5: Japanese Patent Publication (A) No. 2008-121081

SUMMARY OF INVENTION Technical Problem

In recent years, buildings have become larger in size and higher in stories. In particular, when welded structures become larger in size, the steel materials used become larger in size and the required welding efficiency becomes higher. Due to this, the heat input at the time of welding becomes higher. With large heat input welding, the rise in temperature of the HAZ at the time of welding becomes remarkable and the cooling rate falls.

For this reason, coarsening of the grains of the old austenite (below, sometimes referred to as “old γ”) and precipitation of carbides etc. at the old γ-grain boundaries of the HAZ are promoted. As a result, the drop in reheat embrittlement and toughness of the HAZ becomes remarkable.

Further, to raise the high temperature strength of a steel material, after hot rolling, it is preferable to perform accelerated cooling to suppress the formation of bainite. On the other hand, if performing accelerated cooling, due to the temperature control at the time of cooling or nonuniformity of the cooling, the steel material sometimes deforms. Therefore, as the method of production of a steel material, the method of not performing accelerated cooling after hot rolling, but allowing natural cooling is preferable.

However, when allowing natural cooling after hot rolling, a bainite structure becomes difficult to obtain. This is disadvantageous in obtaining high temperature strength. Furthermore, if increasing the amount of addition of alloy elements to secure high temperature strength without accelerated cooling, there was the problem that grain boundary precipitation etc. caused reheat embrittlement of the HAZ.

The present invention was made in consideration of the above problem and has as its object the provision of a fire-resistant steel material superior in HAZ reheat embrittlement resistance and low temperature toughness even in the case of large heat input welding and a method of production of the same.

Solution to Problem

The inventors engaged in detailed studies through experimentation and analysis on the chemical compositions and production conditions for preventing the reheat embrittlement of large heat input HAZ and securing low temperature toughness of the HAZ. As a result, they learned that to secure both reheat embrittlement resistance and low temperature toughness of the HAZ, control of the contents of the C, Mn, Cr, Nb, and Cu is extremely important.

The gist of the present invention, based on this discovery, is as follows:

-   (1) A fire-resistant steel material superior in weld heat affected     zone reheat embrittlement resistance and low temperature toughness     characterized by containing, by mass %, -   C: 0.012% to 0.050%, -   Si: 0.01% to 0.50%, -   Mn: 0.80% to 2.00%, -   Cr: 0.80% to 1.90%, -   Nb: 0.01% to 0.05%, -   N: 0.001% to 0.006%, -   Ti: 0.010% to 0.030%, and -   Al: 0.005% to 0.10%,     further limiting the contents of Cu, Mo, B, P, S, and O to: -   Cu: 0.10% or less, -   Mo: less than 0.01%, -   B: less than 0.0003%, -   P: less than 0.02%, -   S: less than 0.01%, and -   O: less than 0.01%, and     having a balance of Fe and unavoidable impurities, having contents     of C, Mn, Cr, Nb, and Cu [mass %] satisfying

−1200C−20Mn+30Cr−330Nb−120Cu≧−80,

having a steel structure as observed by an optical microscope of an area fraction of 80% or more of a ferrite phase, and having a balance of the steel structure of a bainite phase, martensite phase, and mixed martensite-austenite structure.

-   (2) A fire-resistant steel material superior in weld heat affected     zone reheat embrittlement resistance and low temperature toughness     as set forth in the above (1) characterized by further containing,     by mass %, one or both of -   V: 0.40% or less and -   Ni: 1.00% or less. -   (3) A fire-resistant steel material superior in weld heat affected     zone reheat embrittlement resistance and low temperature toughness     as set forth in the above (1) or (2) characterized by further     containing, by mass %, -   Zr: 0.010% or less, -   Mg: 0.005% or less, -   Ca: 0.005% or less, -   Y: 0.050% or less, -   La: 0.050% or less, and -   Ce: 0.050% or less. -   (4) A method of production of a fire-resistant steel material     superior in weld heat affected zone reheat embrittlement resistance     and low temperature toughness characterized by heating a steel slab     having the steel compositions as set forth in any of the above (1)     to (3) to 1150 to 1300° C. temperature, then hot working or hot     rolling it at 800° C. to 900° C. temperature by a reduction ratio of     50% or more, then allowing it to cool. -   (5) A method of production of a fire-resistant steel material     superior in weld heat affected zone reheat embrittlement resistance     and low temperature toughness characterized by applying the method     of production as set forth in the above (4), then treating the steel     material at a 400° C. to less than 650° C. temperature range for 5     minutes to 360 minutes for tempering heat treatment.

Advantageous Effects of Invention

According to the present invention, a fire-resistant steel material having a high yield strength at 600° C. temperature even if exposed to fire, suppressed in reheat embrittlement at the weld heat affected zone, and superior in low temperature toughness of the base material and weld heat affected zone is obtained. Further, it becomes possible to produce a fire-resistant steel material superior in weld heat affected zone reheat embrittlement resistance and low temperature toughness by a high productivity method of production using the steel as hot rolled.

Therefore, the contribution of the fire-resistant steel material of the present invention to the securing of the safety of buildings using it is extremely great. The contribution to the industry is extremely remarkable.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view showing the effects of C, Mn, Cr, Nb, and Cu on the reheat embrittlement resistance of the HAZ.

EMBODIMENTS OF INVENTION

As one feature of the present invention, the positive use of Cr may be mentioned. Even if adding Cr, this will not contribute much at all to the yield strength or tensile strength at room temperature and the high temperature strength. However, the addition of Cr results in a remarkable alleviation of HAZ reheat embrittlement.

This is considered to be due to the fact that Cr forms several nm to tens of nm clusters of carbides. Due to the formation of fine carbides of Cr, formation of coarse carbides causing embrittlement of the grain boundaries and segregation of C at the grain boundaries are suppressed.

Further, to secure high temperature strength, it is necessary to introduce dislocations into the structure of the steel material. For introduction of dislocations, formation of martensite, bainite, and other hard phases is effective. It is necessary to add certain amounts of C, Mn, and Nb as elements improving the hardenability.

On the other hand, to obtain a sufficient low temperature toughness at the time of large heat input welding, it is necessary to limit the amount of C to a level lower than steel materials for general use, that is, to 0.05% or less. Further, by limiting the amount of C to 0.05% or less, low temperature toughness of the base material can also be secured. Further, the C and Nb contributing to the formation of carbides lower the reheat embrittlement resistance. Further, Cu improves the hardenability, but the HAZ reheat embrittlement becomes remarkable.

Next, B, which forms nitrides at the grain boundaries and causes remarkable reduction of the reheat embrittlement resistance, is limited in content to less than 0.0003% and is preferably not added. For Mo as well, to suppress the precipitation of Mo carbides and Laves phases at the grain boundaries, Mo is not positively added and is restricted in content to less than 0.01%. On the other hand, Ti is effective for alleviation of reheat embrittlement. The reason is that carbides and nitrides of Ti precipitate inside the grains as well whereby the carbides and nitrides precipitating at the grain boundaries are reduced.

Furthermore, the inventors etc. engaged in detailed studies through experimentation and analysis on the effects of the various alloy elements of fire-resistant steel on the HAZ reheat embrittlement. Specifically, they produced fire-resistant steels having various compositions containing C: 0.010 to 0.050%, Si: 0.01 to 0.50%, Mn: 0.80 to 2.00%, Cr: 0.80 to 1.90%, Nb: 0.01 to less than 0.05%, N: 0.001 to 0.006%, Ti: 0.010 to 0.030%, Al: 0.005 to 0.10%, and Cu: 0 to 0.10% and having a balance of Fe. Note that, for the method of production, a process not involving accelerated cooling, but allowing natural cooling after hot rolling was employed.

Test pieces were obtained from the obtained fire-resistant steels and subjected to heat cycles envisioning welding by a heat input of 10 kJ/mm. The “heat cycle envisioning welding by a heat input of 10 kJ/mm” is a heat history of heating from room temperature to 1400° C. by 20° C./s, holding at 1400° C. for 2 seconds, then cooling during which making the cooling rate from 800° C. to 500° C. 3° C./s. After that, the test pieces were raised from room temperature to 600° C. temperature over 60 minutes, held at 600° C. for 30 minutes, then subjected to tensile tests at 600° C. and measured for the area reduction of the broken parts of the test pieces. The area reduction values were used as indicators of the HAZ reheat embrittlement. 20% or more was considered good.

As a result, by multiple linear regression analysis, it was learned that the HAZ reheat embrittlement resistance can be defined by −1200C−20Mn+30Cr−330Nb−120Cu. Further, as shown in FIG. 1, it was learned that to secure HAZ reheat embrittlement resistance, it is necessary that the contents of C, Mn, Cr, Nb, and Cu satisfy the following formula using the contents of the elements (mass %):

−1200C−20Mn+30Cr−330Nb−120Cu≧−80

Note that, when Cu is not included, the elements must satisfy

−1200C−20Mn+30Cr−330Nb≧−80.

Here, an upper limit of −1200C−20Mn+30Cr−330Nb−120Cu is not set since the higher the value, the better the HAZ reheat embrittlement resistance. However, based on the lower limit values of the contents of C, Mn, Nb, and Cu and the upper limit value of the content of Cr, the upper limit of −1200C−20Mn+30Cr−330Nb−120Cu becomes 23.3.

As explained above, in particular, by controlling the amounts of addition of C, Mn, Cr, Nb, Ti, Cu, Mo, and B, it is possible to secure high temperature strength of the base material and achieve both reheat embrittlement resistance and low temperature toughness of the HAZ at the time of large heat input welding.

Further, with the system of compositions of the present invention, by performing 800° C. or more hot rolling or hot working, then allowing the steel to naturally cool, a fire-resistant steel material having a room temperature tensile strength of 400 MPa to 610 MPa is obtained. In particular, the yield stress at 600° C. temperature becomes 157 MPa or more when the room temperature tensile strength is 400 to 489 MPa in range and becomes 217 MPa or more when the room temperature tensile strength is 490 to 610 MPa in range.

Further, after the process of natural cooling to room temperature after hot rolling, by tempering at 400° C. to 650° C. temperature, it becomes possible to lower only the room temperature tensile strength without lowering the high temperature strength and to improve the low temperature toughness of the base material.

Below, the present invention will be explained in detail.

First, the reasons for limitation of the ranges of essential chemical compositions defined for working the present invention will be explained. Note that, in the following explanation, the amounts of addition of the elements are all expressed by mass %.

[C: 0.012% to 0.050%]

C is an element effective for improving the hardenability of the steel material and is added in an amount of 0.012% or more. Note that, from the viewpoint of sufficiently securing hardenability, addition of 0.015% or more or 0.020% or more is more preferable. On the other hand, if adding over 0.050% of C, at the HAZ at the time of large heat input welding, many mixed martensite-austenite structures (below, sometimes called “MA phases”) or precipitated carbides are formed. As a result, sometimes the low temperature toughness of the HAZ is remarkably degraded. In addition, sometimes the amount of carbides precipitating at the grain boundaries of the HAZ at the time of a fire is increased and reheat embrittlement of the HAZ is invited. For this reason, the range of addition of C was defined as 0.012% to 0.050%. To secure strength, C is preferably added in an amount of 0.020% or more. On the other hand, to raise the low temperature toughness of the HAZ, the upper limit of the amount of C is preferably set to 0.040% or less.

[Si: 0.01% to 0.50%]

Si is a deoxidizing element and an element contributing to the improvement of the hardenability. At least 0.01% or more is added. On the other hand, if adding Si over 0.50%, sometimes the amount of formation of the MA phase of the HAZ at the time of large heat input welding is increased and the low temperature toughness is reduced. For this reason, the range of addition of Si is defined as 0.01% to 0.50%. To raise the strength, addition of 0.05% or more of Si is preferable. Further, to raise the toughness of the HAZ, making the upper limit of the amount of Si 0.30% or less is preferable.

[Mn: 0.80% to 2.00%]

Mn is effective for improving the hardenability. To secure the 400 MPa or more room temperature tensile strength aimed at by the present invention, addition of 0.80% or more is necessary. On the other hand, Mn is liable to segregate at the grain boundaries and aggravate HAZ reheat embrittlement, so the upper limit of the amount of addition was set at 2.00%. To raise the strength, addition of 1.00% or more of Mn is preferable. On the other hand, to secure HAZ reheat embrittlement resistance, the upper limit of the amount of Mn is preferably made 1.60% or less. To raise the HAZ low temperature toughness, the upper limit of the amount of Mn is preferably set to 1.50% or less.

[Cr: 0.80% to 1.90%]

Cr does not contribute much at all to the room temperature yield strength and tensile strength and, further, does not contribute much at all to the improvement of the high temperature strength when using materials of the system of chemical compositions of the present invention to produce a steel material as hot rolled. This was found by research of the inventors. On the other hand, Cr forms fine Cr carbides and thereby consumes carbon atoms without itself contributing to HAZ reheat embrittlement and has the effect of suppressing HAZ reheat embrittlement due to coarsening of Nb or V carbides.

In the present invention, in particular, to suppress reheat embrittlement, 0.80% or more of Cr is added. The preferable lower limit of the amount of Cr is 0.90% or more, while the more preferable lower limit is 1.00% or more. Further, if adding Cr over 1.90%, the HAZ toughness falls due to the hardening of the HAZ and the increase of the MA phase, so the upper limit is made 1.90%. The preferable upper limit of the amount of Cr is 1.80% or less and the more preferable upper limit is 1.50% or less.

Note that, in the present invention, the greater the amounts of elements such as C, Mn, Nb, Ni, and Cu aggravating HAZ reheat embrittlement added, the more preferable it is to increase the amount of addition of Cr to counter this.

[Nb: 0.01% to less than 0.05%]

Nb increases the hardenability of the steel material and contributes to the improvement of the dislocation density as well. It precipitates as carbides or nitrides and contributes to the improvement of the room temperature tensile strength and high temperature strength as well, so 0.01% or more is added. However, if adding 0.05% or more of Nb, the drop in the HAZ toughness and the HAZ reheat embrittlement caused by the coarse precipitation of NbC at the grain boundaries become remarkable, so the amount of addition is restricted to 0.01% to less than 0.05%. To raise the room temperature tensile strength, it is preferable to add Nb in an amount of 0.02% or more. On the other hand, to suppress a drop in the HAZ toughness and reheat embrittlement resistance, it is preferable to make the upper limit of the amount of Nb less than 0.03%.

[N: 0.001% to 0.006%]

N forms nitrides with various types of alloy elements to contribute to the improvement of the high temperature strength, so 0.001% or more is added. The preferable lower limit of the amount of N is 0.002% or more, while the more preferable one is 0.003% or more. However, if adding a large amount of N, the nitrides precipitating at the grain boundaries of the HAZ become coarser and the HAZ reheat embrittlement becomes remarkable, so the upper limit was made 0.006%. The preferable upper limit of the amount of N is 0.005% or less.

[Ti: 0.010% to 0.030%]

Ti precipitates as carbides and nitrides and contributes to the improvement of the room temperature tensile strength and high temperature strength. Further, Ti precipitates in the HAZ as carbides and nitrides not only at the grain boundaries, but also inside the grains and thereby consumes the carbon and nitrogen. As a result, Ti suppresses the coarse precipitation of carbides or nitrides of other, alloy elements at the grain boundaries and contributes to the suppression of HAZ reheat embrittlement. To obtain these effects, addition of 0.010% or more of Ti is necessary. The preferable lower limit of the amount of Ti is 0.015% or more, while the more preferable lower limit is 0.020%. On the other hand, if adding Ti over 0.030%, the base material remarkably falls in low temperature toughness, so the upper limit was made 0.030%. The preferable upper limit of the amount of Ti is 0.025% or less.

[Al: 0.005% to 0.10%]

Al is an element required for deoxidation of the steel material. In particular, in a steel material containing Cr, Al is added as a main deoxidizing element to prevent oxidation of the Cr during the refining. This effect of enabling control of the concentration of oxygen in the molten steel is obtained by addition of 0.005% or more, so the lower limit value of Al was made 0.005%. The preferable lower limit of the amount of Al is 0.020% or more, while the more preferable one is 0.030% or more. On the other hand, if the content of Al exceeds 0.10%, coarse oxide clusters are formed and the toughness of the steel material is impaired in some cases, so the upper limit value was set at 0.10%. The preferable upper limit of the amount of Al is 0.075% or less, while the more preferable upper limit is 0.050% or less.

[Cu: 0.10% or less]

Cu is effective for improvement of the room temperature tensile strength and high temperature strength by the improvement of the hardenability, but in the present invention is an element causing remarkable HAZ reheat embrittlement. Therefore, while inclusion of a small amount due to factors in industrial production is unavoidable, it is preferable to refrain from deliberate addition. The allowable upper limit is set to 0.10%. The amount of Cu is preferably limited to 0.05% or less.

[Mo: less than 0.01%]

Mo contributes to the improvement of the room temperature tensile strength and high temperature strength by improvement of the hardenability and precipitation strengthening. However, Mo easily coarsely precipitates as carbides or Laves phases at the HAZ grain boundaries and results in remarkable HAZ reheat embrittlement, so addition of Mo is not preferable in the present invention. Therefore, while inclusion of a small amount due to factors in industrial production is unavoidable, it is preferable to refrain from deliberate addition. From the leeway in industrial production, the upper limit of the amount of addition is set to less than 0.01%.

[B: less than 0.0003%]

B contributes to the improvement of the room temperature tensile strength and high temperature strength by improvement of the hardenability and precipitation strengthening. However, B nitrides easily coarsely precipitate at the HAZ grain boundaries and result in remarkable HAZ reheat embrittlement, so addition of B is not preferable in the present invention. Therefore, while inclusion of a small amount due to factors in industrial production is unavoidable, it is preferable to refrain from deliberate addition. From the leeway in industrial production, the upper limit of the amount of addition is set to less than 0.0003%.

[P: less than 0.02%]

P is an impurity which remarkably reduces the low temperature toughness of the base material and further also results in remarkable HAZ reheat embrittlement at the time of a fire, so the upper limit of the amount of addition is set to less than 0.020%. The preferable upper limit of the amount of P is 0.01% or less.

[S: less than 0.01%]

S is an impurity which remarkably reduces the low temperature toughness of the base material and further also results in remarkable HAZ reheat embrittlement at the time of a fire, so the upper limit of the amount of addition is set to less than 0.01%. The preferable upper limit of the amount of S is 0.005% or less.

[O: less than 0.01%]

O is an impurity which remarkably reduces the low temperature toughness of the base material and further also results in remarkable HAZ reheat embrittlement at the time of a fire, so the upper limit of the amount of addition is set to less than 0.010%. The preferable upper limit of the amount of O is 0.005% or less, while the more preferable limit is 0.003% or less.

In the present invention, in addition to the above essential elements, the elements explained below may further be selectively added.

Below, the reasons for limitation of the ranges of addition of the optional elements in the present invention will be explained.

[V: 0.40% or less]

V forms carbides due to reheating at the time of a fire and thereby is extremely effective for improving the high temperature strength, so addition of 0.03% or more is preferable. On the other hand, if adding over 0.40% of V, the carbides precipitating at the grain boundaries of the HAZ will coarsen and remarkable HAZ reheat embrittlement will result, so the amount of addition is preferably limited to 0.40% or less. Further, the amount of addition of V is more preferably 0.05% to 0.20% in range.

[Ni: 1.00% or less]

Ni is effective for improvement of the room temperature tensile strength and high temperature strength by the improvement of the hardenability, but causes remarkable HAZ reheat embrittlement. Therefore, while inclusion of a small amount due to factors in industrial production is unavoidable, it is preferable to refrain from deliberate addition. The allowable upper limit is set to 1.00%. The preferable upper limit of the amount of Ni is 0.40% or less, while the more preferable limit is 0.20% or less.

[Zr: 0.010% or less]

Zr precipitates as carbides and nitrides and contributes to the improvement of the room temperature tensile strength and high temperature strength. To obtain this effect, 0.002% or more of Zr is preferably added. On the other hand, if adding over 0.010% of Zr, the carbides precipitating at the grain boundaries coarse and the HAZ reheat embrittlement becomes remarkable, so the upper limit of the amount of addition of Zr is preferably made 0.010% or less. The preferable upper limit of the amount of Zr is 0.005% or less.

[Mg: 0.005% or less]

Mg controls the form of the sulfides in the steel material and has the effect of reducing the drop in base material toughness due to sulfides. To obtain such an effect, 0.0005% or more of Mg is preferably added. On the other hand, even if adding over 0.005% of Mg, the effect becomes saturated, so when adding Mg, the upper limit is preferably made 0.005% or less. The preferable upper limit of the amount of Mg is 0.002% or less.

[Ca: 0.005% or less]

Ca controls the form of the sulfides in the steel material and has the effect of reducing the drop in base material toughness due to sulfides. To obtain such an effect, 0.0005% or more of Ca is preferably added. On the other hand, if adding over 0.005% of Ca, the effect becomes saturated, so when adding Ca, the upper limit is preferably made 0.005% or less. The preferable upper limit of the amount of Ca is 0.003% or less.

[Y: 0.050% or less]

Y controls the form of the sulfides in the steel material and has the effect of reducing the drop in base material toughness due to sulfides. To obtain such an effect, 0.001% or more of Y is preferably added. On the other hand, if adding over 0.050% of Y, the effect becomes saturated, so when adding Y, the upper limit is preferably made 0.050% or less. The preferable upper limit of the amount of Y is 0.030% or less.

[La: 0.050% or less]

La controls the form of the sulfides in the steel material and has the effect of reducing the drop in base material toughness due to sulfides. To obtain such an effect, 0.001% or more of La is preferably added. On the other hand, if adding over 0.050% of La, the effect becomes saturated, so when adding La, the upper limit is preferably made 0.050% or less. The preferable upper limit of the amount of La is 0.020% or less.

[Ce: 0.050% or less]

Ce controls the form of the sulfides in the steel material and has the effect of reducing the drop in base material toughness due to sulfides. To obtain such an effect, 0.001% or more of Ce is preferably added. On the other hand, if adding over 0.050% of Ce, the effect becomes saturated, so when adding Ce, the upper limit is preferably made 0.050% or less. The preferable upper limit of the amount of Ce is 0.020% or less.

In the present invention, due to the above limits on the chemical compositions, a fire-resistant steel material having a high yield strength at 600° C. temperature even when exposed to fire and simultaneously suppressed in reheat embrittlement of the heat affected zone of the weld joint and superior in base material and weld joint low temperature toughness can be realized.

Next, the structure of the steel material of the present invention will be explained.

In general, a steel material may be considered to exhibit high temperature strength due to dislocation strengthening due to dislocations present in the steel material and precipitates blocking movement of dislocations. Therefore, when the temperature of the steel material exceeds 550° C. and dislocations merge and are eliminated due to upward movement of the dislocations, sometimes the high temperature strength rapidly declines.

For this reason, to secure a high temperature strength, it is effective that the steel material have a sufficient margin of dislocations at the time before being exposed to fire, that is, at room temperature, or include large amounts of structures blocking movement of dislocations, specifically precipitates and crystal grain boundaries.

Further, while explained in detail in the method of production explained later, in the present invention, from the viewpoint of the productivity of products with stable mechanical properties, the fire-resistant steel material is produced as hot rolled without using accelerated cooling. For this reason, the steel material structure (metal structure), as observed by an optical microscope, is a structure having an area fraction of 80% or more of a ferrite phase and a balance of a bainite phase, martensite phase, and mixed martensite-austenite structures (MA phase). To secure the toughness of the base material, the area fraction of the ferrite phase is preferably made 85% or more. Further, to secure strength, the area fraction of the ferrite phase is preferably made 97% or less.

A steel material having the chemical composition of the present invention and having a steel structure made the above structure, as explained in detail later, is hot worked or hot rolled at 800° C. to 900° C. temperature by a large reduction ratio. Due to such production conditions, the precipitates blocking dislocations in the steel material are made to finely disperse and, further, the structure can be made a finer grain, so a greater high temperature strength is obtained.

Next, the mechanical properties of the steel material of the present invention will be explained.

In the fire-resistant steel material of the present invention, by applying to the steel material of the above steel compositions and steel structure the various steps of the conditions shown in the method of production explained below, it becomes possible to provide fire-resistant steel plate having the mechanical properties explained below.

[Room Temperature Tensile Strength and 600° C. Yield Stress]

In the fire-resistant steel material of the present invention, the room temperature tensile strength is 400 MPa to 610 MPa. The yield stress at 600° C. temperature is 157 MPa or more when the room temperature tensile strength is 400 MPa to 489 MPa and is 217 MPa or more when the room temperature tensile strength is 490 MPa to 610 MPa. Due to this, in building applications, a fire-resistant steel material securing the various requirements in building design and having a sufficient margin of safety in fires can be realized.

[600° C. Breakage Area Reduction Value]

In the fire-resistant steel material of the present invention, the reheat embrittlement resistance is evaluated by using a test piece given a heat history envisioning welding by a heat input of 5 kJ/mm and 10 kJ/mm and measuring the area reduction value of the broken part at 600° C. temperature. In the present invention, a fire-resistant steel material having an area reduction value of the broken part at a 600° C. temperature of 20% or more is obtained. Due to this, a fire-resistant steel material having sufficient deformation ability when the weld joint HAZ is reheated to the envisioned temperature 600° C. of a fire can be realized.

[Method of Production of Fire-Resistant Steel Material]

Below, the reasons for limitation of the method of production of a fire-resistant steel material of the present invention superior in high temperature strength of the base material and reheat embrittlement resistance and low temperature toughness of the weld heat affected zone will be explained.

The method of production of a fire-resistant steel material of the present invention is a method heating a steel slab having the above-mentioned steel compositions to 1150° C. to 1300° C. temperature, then hot working or hot rolling it at a 800° C. to 900° C. temperature by a reduction ratio of 50% or more, then allowing it to cool.

In the method of production of the present invention, to secure the requirements in building design and obtain a sufficient safety margin against fire in a fire-resistant steel material used for building applications, as explained above, a steel slab having a chemical composition having as its required conditions giving a room temperature tensile strength of 400 MPa to 610 MPa, a high yield strength at 600° C., an area reduction value of the broken part at 600° C. of the weld HAZ of the steel material of 20% or more, a superior reheat embrittlement resistance, secured low temperature toughness even at the HAZ due to welding by a heat input of 5 kJ/mm, and secured base material toughness is used as the material. Further, the steel slab may be hot worked or hot rolled at a prescribed temperature and reduction amount to produce a fire-resistant steel material satisfying all of the above properties.

[Reduction Ratio in Hot Working or Hot Rolling]

As explained above, the high temperature strength of a steel material is considered to be achieved by dislocation strengthening by dislocations present in the steel material and the precipitates blocking movement of dislocations, so if the temperature exceeds 550° C. and the dislocations merge and are eliminated due to upward movement of the dislocations, the high temperature strength sometimes is reduced. Therefore, to secure a good high temperature strength, it is effective to provide a sufficient extra margin of amount of dislocations at room temperature or include a large number of precipitates, crystal grain boundaries, or other structures blocking movement of dislocations.

Here, in the method of production of the present invention, from the viewpoint of productivity of a product with stable mechanical properties in actual production, the object is to produce a fire-resistant steel material as hot rolled without using accelerated cooling. Therefore, the steel structure as a whole does not become a high dislocation density bainite or martensite. The steel structure becomes one with the low dislocation density ferrite structures accounting for 80% or more of the area fraction of the steel structure as observed by an optical microscope and with a balance of less than 20% of bainite, martensite, and MA.

Therefore, to secure a high high-temperature strength in the present invention, relying on just the increase of the fraction of bainite or martensite in the steel material is insufficient. It is necessary to make the precipitates blocking dislocation finely disperse and make the structure finer in grain.

The inventors discovered by experimentation and analysis that to realize fine dispersion of precipitates in the steel material and finer grain of the structure, when hot rolling steel slabs having the chemical composition of the present invention, it is effective to increase the reduction ratio at a 800° C. to 900° C. temperature, specifically, making the reduction ratio 50% or more, more preferably 70% or more.

Further, by introducing a large amount of dislocations at the temperature region right before transformation from austenite to ferrite or bainite, these dislocations will become sites for formation of nuclei for precipitation and these dislocations will become sites for formation of nuclei for ferrite or bainite transformation. Due to this, it was learned that it is possible to realize both fine dispersion of precipitates and finer grains of the structure.

Note that, in general, if the amount of reduction is made large in the austenite region, the higher transformation temperature will cause the bainite fraction to fall and the ferrite fraction to rise in some cases, but in the chemical compositions of the present invention, the amount of C is kept low, so it is clear that bainite transformation easily occurs and a drop in the bainite fraction can be suppressed.

[Heating Temperature Before Hot Working or Hot Rolling]

As explained above, in the method of production of the present invention, effective utilization of the precipitation of the alloy elements is important. As the means for stably and reliably obtaining precipitation of such alloy elements, it is necessary to heat the steel slabs before hot working or hot rolling to 1150° C. to 1300° C. Such heat treatment heats the steel slabs to a 1150° C. or more temperature to thereby cause the carbides or nitrides of the various alloy elements, for example, NbC, NbN, VC, TiC, ZrC, Cr₂₃C₆, etc., to completely or as much as possible form solid solutions and thereby improve the hardenability after hot rolling and increase the amount of precipitation after hot working or hot rolling.

When not heating before hot working or hot rolling, the C, Cr, Nb, V, Ti, Zr, and other alloy elements already precipitate as coarse precipitates before hot rolling, so reduction of the dislocation density of the steel material due to the drop in hardenability after hot working or hot rolling or reduction of the amount of precipitation strengthening due to the reduction in fine carbides or nitrides precipitating after hot working or hot rolling is sometimes invited.

On the other hand, if making the heating temperature before hot working or hot rolling over 1300° C., the increase in oxide scale at the surface of the steel material becomes remarkable, so the upper limit of the heating temperature is set to 1300° C.

[Tempering Heat Treatment]

In the method of production of the present invention, it is also possible to allow the steel material to naturally cool to room temperature after hot rolling, then apply a step of tempering heat treatment to the steel material. By applying tempering heat treatment to the steel material, it becomes possible to promote the precipitation of alloy elements remaining in the solid solution state without completely precipitating by just the natural cooling after hot rolling and further increase the number of precipitates suppressing the reduction in dislocations at the time of fire.

In such tempering treatment, the temperature can be determined by suitable selection between 400° C. to 650° C. By deciding this based on the required room temperature tensile strength and type of precipitated alloy elements, the effect of the present invention can be further enhanced.

Further, the same applies to the time of the tempering heat treatment. When the change in structure at the time of tempering is governed by the dispersion of the substance, raising the temperature and prolonging the time have similar effects, so the time can be suitably determined between 5 minutes to 30 minutes in accordance with the tempering temperature.

As explained above, the method of production of a fire-resistant steel material of the present invention heats a steel slab having steel compositions of the above defined range to 1150° C. to 1300° C. temperature, then hot works or hot rolls it at 800° C. to 900° C. temperature by a reduction ratio of 50% or more, then allows the result to naturally cool. According to this method of production, it is possible to produce a fire-resistant steel material having a high yield strength at a 600° C. temperature even when exposed to fire, simultaneously suppressed in reheat embrittlement at the heat affected zone of the weld joint, and giving superior low temperature toughness of the base material and weld joint. Therefore, it becomes possible to produce a fire-resistant steel material for buildings superior in high temperature strength and superior in weld joint reheat embrittlement resistance by an economical compositions using less alloy elements and a high productivity method of production stopping at hot rolling.

Examples

Below, examples of the fire-resistant steel material of the present invention and method of production of the same will be given and the present invention explained more specifically, but the present invention is of course not limited to the following examples and can be worked suitably changed within a scope complying with the gist of the invention explained before and after this. These are all included in the technical scope of the present invention.

[Preparation of Fire-Resistant Steel Material]

The deoxidation and desulfurization and the chemical compositions of the molten steel in the steelmaking process were controlled and the steel continuously cast to prepare slabs of the chemical compositions shown in the following Table 1. Further, under the production conditions shown in Table 2, the slabs were reheated and hot worked to the respective plate thicknesses, then heat treated under different conditions to prepare fire-resistant steel materials of the invention examples and comparative examples.

Specifically, first, the slabs were reheated at 1150° C. to 1300° C. temperatures for 1 hour, then immediately started to be rough rolled to obtain steel plates of plate thicknesses of 100 mm at a 1050° C. temperature. Further, under the conditions shown in Table 2, they were made into thick-gauge steel plates of final thicknesses of 15 mm to 35 mm or were forged or rolled into steel shapes of complicated cross-sectional shapes with maximum thicknesses of 15 mm to 35 mm. The finishing temperatures were controlled to 800° C. or more. The reduction ratios at the 800° C. to 900° C. temperature at that time were controlled to the values shown in Table 1 while performing final rolling. Further, after the end of the rolling, the plates were immediately allowed to cool to thereby prepare the fire-resistant steel materials of the invention examples and comparative examples.

[Evaluation and Testing]

The fire-resistant steel materials of the invention examples and comparative examples prepared by the above method were evaluated and tested as follows:

First, regarding the room temperature tensile test, this was performed based on JIS Z 2241. When an upperyield point appeared on the stress-strain curve, the upper yield point was defined as the room temperature yield strength, while when one did not appear, the 0.2% yield strength was defined as the room temperature yield strength.

Further, regarding the high temperature tensile test, this was performed based on JIS G 0567 at a 600° C. temperature. The measured 0.2% yield strength was made the 600° C. yield strength.

Further, the 600° C. tensile area reduction value of the HAZ (weld heat affected zone) was evaluated by a heat cycle applying a heat history envisioning a heat input of 5 kJ/mm and 10 kJ/mm to the steel slab. After applying the heat cycle, the slab was raised from room temperature to 600° C. temperature over 60 minutes, was held at 600° C. for 30 minutes, then was subjected to a tensile test at 600° C. The area reduction value of the broken part of the test piece was measured and used as an indicator of reheat embrittlement of the HAZ. The threshold value of the indicator was made 20% or more.

Further, the Charpy test of the base material was performed by taking a 2 mm V-shaped impact test piece from the plate thickness ½ t of each steel material based on JIS Z 2202 and performing an impact test based on the method of JIS Z 2242. At this time, the threshold value of the absorption energy was made 27J considering the earthquake resistance of building structures.

Further, the Charpy test of the HAZ was performed by applying to each steel material a heat cycle envisioning welding by a heat input of 5 kJ/mm and a heat input of 10 kJ/mm, then taking a 2 mm V-shaped impact test piece based on JIS Z 2202 and performing an impact test based on the method of JIS Z 2242. At this time, the threshold value of the absorption energy was made 27J considering the earthquake resistance of building structures.

Note that, the “heat history envisioning welding by a heat input of 5 kJ/mm” is a heat cycle of heating from room temperature to 1400° C. by 20° C./s, holding at 1400° C. for 1 second, then cooling during which cooling from 800° C. to 500° C. in range by 15° C./s. Further, the “heat history envisioning welding by a heat input of 10 kJ/mm” is a heat cycle of heating from room temperature to 1400° C. by 20° C./s, holding at 1400° C. for 2 seconds, then cooling during which cooling from 800° C. to 500° C. in range by 3° C./s.

Further, regarding the structure of the steel material, the structure of the steel material was observed by an optical microscope. From the results, the total of the area fractions of bainite, martensite, and MA was calculated and the area fraction of ferrite was found.

A list of the chemical compositions of the fire-resistant steel materials of the invention examples and comparative examples is shown in the following Table 1 and a list of the production conditions and mechanical properties of the steel materials is shown in the following Table 2.

Note that, in Table 1, Steel Type Nos. 1 to 21 are invention examples having steel compositions defined by the present invention, while Steel Type Nos. 22 to 34 are comparative examples given steel compositions outside the defined range of the present invention. The value of formula: −1200C−20Mn+30Cr−330Nb−120Cu is shown as the HAZ reheat embrittlement coefficient.

Further, in Table 2, the produced plate thickness, heating temperature, hot rolling conditions (finishing temperature, reduction ratio), tempering temperature, room temperature tensile strength (room temperature TS), room temperature yield strength (room temperature YS), 600° C. yield strength (600° C. YS), HAZ 600° C. tensile test breakage area reduction value (HAZ reheat embrittlement area reduction value), 0° C. base material Charpy absorption energy, and 0° C. HAZ Charpy absorption energy are shown.

Further, in Table 2, regarding the strength level, steels of a room temperature tensile strength of 400 to 489 MPa were displayed as the 400 MPa class, while steels of a room temperature tensile strength of 490 to 610 MPa were displayed as the 500 MPa class.

Further, in Table 1 and Table 2, values outside the range of the present invention are shown underlined.

TABLE 1 Steel type Chemical composition, (mass %) no. C Si Mn P S Cr Nb Ti N Al 1 0.049 0.05 0.90 0.005 0.004 1.50 0.020 0.011 0.0040 0.050 2 0.050 0.45 1.00 0.005 0.004 1.80 0.020 0.025 0.0039 0.049 3 0.040 0.30 1.01 0.007 0.005 1.30 0.031 0.011 0.0039 0.049 4 0.039 0.30 1.00 0.010 0.005 1.90 0.030 0.010 0.0039 0.049 5 0.031 0.29 1.20 0.010 0.006 1.30 0.010 0.019 0.0030 0.040 6 0.030 0.29 1.21 0.005 0.006 1.50 0.010 0.019 0.0030 0.040 7 0.030 0.29 1.55 0.005 0.003 1.01 0.023 0.021 0.0031 0.040 8 0.029 0.30 1.54 0.006 0.003 1.01 0.020 0.021 0.0032 0.045 9 0.030 0.30 1.56 0.007 0.003 1.00 0.021 0.025 0.0029 0.045 10 0.030 0.30 1.56 0.004 0.004 1.00 0.020 0.025 0.0028 0.045 11 0.030 0.30 1.40 0.004 0.007 1.31 0.033 0.021 0.0029 0.035 12 0.031 0.30 1.39 0.004 0.007 1.50 0.032 0.020 0.0029 0.035 13 0.025 0.29 1.35 0.006 0.004 1.00 0.031 0.010 0.0044 0.020 14 0.025 0.05 1.35 0.010 0.004 0.99 0.030 0.010 0.0045 0.020 15 0.024 0.29 1.20 0.012 0.003 0.90 0.048 0.011 0.0044 0.020 16 0.024 0.29 1.20 0.004 0.005 0.89 0.049 0.027 0.0048 0.020 17 0.020 0.10 1.50 0.007 0.005 0.81 0.048 0.028 0.0050 0.071 18 0.020 0.12 1.52 0.003 0.005 0.80 0.040 0.028 0.0051 0.073 19 0.019 0.10 0.82 0.004 0.004 1.02 0.040 0.012 0.0055 0.075 20 0.015 0.11 1.20 0.005 0.005 1.02 0.041 0.024 0.0050 0.080 21 0.012 0.05 1.80 0.005 0.005 1.50 0.040 0.028 0.0019 0.080 22 0.065 0.05 1.00 0.006 0.006 1.00 0.040 0.020 0.0040 0.050 23 0.003 0.30 1.20 0.006 0.006 1.02 0.021 0.011 0.0040 0.050 24 0.030 0.78 0.85 0.006 0.006 0.80 0.021 0.020 0.0030 0.049 25 0.029 0.09 0.45 0.005 0.006 1.20 0.020 0.015 0.0029 0.048 26 0.030 0.25 2.50 0.006 0.006 0.82 0.049 0.012 0.0031 0.040 27 0.025 0.20 1.40 0.005 0.005 0.44 0.048 0.010 0.0030 0.041 28 0.030 0.20 1.40 0.004 0.002 2.30 0.040 0.012 0.0040 0.041 29 0.040 0.25 1.20 0.005 0.003 0.81 0.060 0.011 0.0031 0.040 30 0.025 0.30 1.55 0.005 0.004 0.99 0.150 0.025 0.0029 0.042 31 0.030 0.31 1.56 0.004 0.003 1.30 0.030 0.040 0.0029 0.030 32 0.030 0.25 1.40 0.004 0.002 1.01 0.030 0.021 0.0020 0.030 33 0.025 0.29 1.55 0.006 0.005 1.00 0.031 0.013 0.0020 0.050 34 0.029 0.28 1.54 0.006 0.006 1.01 0.030 0.020 0.0021 0.050 35 0.045 0.30 1.85 0.005 0.004 0.81 0.048 0.010 0.0033 0.040 36 0.020 0.31 1.09 0.005 0.004 1.90 0.250 0.010 0.0030 0.040 Steel HAZ reheat type Chemical composition, (mass %) embrittlement no. V Ni Cu Mo B O Other coefficient Remarks 1 0.20 0.0010 Zr 0.005 −38.4 2 0.34 0.0010 Zr 0.005 −32.6 3 0.11 0.0011 −39.4 4 0.12 0.0002 0.0012 Mg 0.002 −19.7 5 0.15 0.10 0.0013 −37.5 6 0.20 0.0011 −18.5 7 0.09 0.0011 −44.3 8 0.09 0.009 0.0013 −41.9 9 0.05 0.0014 −44.1 10 0.05 0.10 0.05 0.0002 0.0015 −49.8 Inv. ex. 11 0.10 0.0002 0.0020 Y 0.030 −35.6 12 0.10 0.09 0.0025 Y 0.030 −41.4 13 0.0024 −37.2 14 0.05 0.003 0.0024 −37.2 15 0.0019 −41.6 16 0.20 0.08 0.0019 Ce 0.015 −51.9 17 0.20 0.10 0.0024 Ca 0.002 −57.5 18 0.20 0.10 0.003 0.0025 Ca 0.002 −55.6 19 0.10 0.004 0.0025 −33.8 20 0.10 0.0001 0.0029 −36.9 21 0.35 0.09 0.009 0.0002 0.0040 La 0.020 −29.4 22 0.09 0.40 0.0020 −81.2 23 0.20 0.003 0.0010 −3.9 24 0.09 0.05 0.005 0.0010 −41.9 25 0.10 0.0012 −14.4 26 0.0013 −77.6 Comp. ex. 27 0.15 0.08 0.0014 −70.2 28 0.10 0.10 0.0020 −20.2 29 0.10 0.10 0.0020 −79.5 30 0.10 0.0015 −80.8 31 0.20 0.20 0.09 0.0030 −48.9 32 0.51 0.0032 −43.6 33 0.050 0.0033 −41.2 34 0.09 0.004 0.0019 0.0035 Ca 0.003 −45.2 35 0.10 0.05 0.0020 −88.5 36 0.11 0.0030 −71.3

TABLE 2 Produced Ord. Ord. Base Steel Strength plate Finishing Red. Ferrite temp. temp. material type level thickness Heating temp. ratio Tempering phase YS TS toughness no. [MPa] [mm] [° C.] [° C.] [%] [° C. × min] [%] [MPa] [MPa] vE 0° C. [J] 1 400 30 1220 850 60 91 271 450 267 2 500 30 1220 850 60 86 330 531 89 3 400 30 1220 850 60 92 307 475 288 4 400 30 1220 850 60 93 237 483 303 5 400 30 1220 850 60 93 297 487 192 6 400 30 1220 850 60 90 278 476 186 7 500 20 1250 840 75 87 420 574 162 8 500 20 1250 840 75 86 420 580 146 9 500 20 1250 840 75 85 454 585 158 10 500 20 1250 840 75 550° C. × 30 min 85 455 582 165 11 500 20 1250 840 75 550° C. × 30 min 88 383 578 201 12 500 20 1250 840 75 86 366 581 170 13 400 16 1180 840 75 95 352 482 284 14 400 16 1180 840 75 94 349 485 291 15 400 16 1180 840 75 96 367 473 299 16 500 12 1220 810 75 86 414 538 130 17 500 12 1220 810 75 600° C. × 30 min 88 479 579 172 18 500 12 1220 810 75 600° C. × 30 min 87 479 580 149 19 400 30 1250 870 60 95 355 420 289 20 400 30 1250 870 60 97 376 464 176 21 500 16 1250 810 75 550° C. × 45 min 88 415 597 124 22 500 30 1220 850 60 580° C. × 30 min 83 469 589 248 23 400 30 1220 850 60 99 339 460 331 24 400 30 1220 850 60 96 425 488 188 25 400 30 1220 850 60 97 360 397 320 26 500 20 1220 840 75 620° C. × 30 min 87 388 599 206 27 500 20 1220 840 75 86 469 564 247 28 500 20 1220 840 75 85 410 597 230 29 500 20 1220 840 75 85 424 576 268 30 500 20 1220 840 75 620° C. × 30 min 82 449 600 154 31 500 20 1220 840 75 620° C. × 30 min 81 450 602 7 32 500 20 1220 840 75 600° C. × 30 min 83 419 580 124 33 500 20 1220 840 75 82 422 600 203 34 500 20 1220 840 75 85 438 593 165 35 500 20 1220 870 75 82 444 605 240 36 500 20 1220 870 75 84 427 580 259 Steel HAZ toughness 600° C. HAZ reheat area type vE 0 [J] YS reduction value [%] no. 5 kJ/mm 10 kJ/mm [MPa] 5 kJ/mm 10 kJ/mm Remarks 1 369 90 197 26 26 Invention 2 326 30 234 21 22 examples 3 346 99 180 31 30 4 340 157 183 38 35 5 337 145 190 31 29 6 362 55 193 44 40 7 311 53 218 30 28 8 307 61 234 23 22 9 295 73 228 26 26 10 294 68 234 20 22 11 311 84 221 28 30 12 294 150 224 26 26 13 346 119 168 48 44 14 355 139 183 42 41 15 361 112 161 48 47 16 331 29 219 24 20 17 291 82 219 36 37 18 293 66 225 35 35 19 372 167 160 54 49 20 363 178 161 66 60 21 299 108 218 64 61 22 286 7 241 7 6 Comparative 23 368 38 144 75 71 examples 24 352 16 167 42 40 25 382 39 149 67 60 26 286 202 224 15 13 27 299 37 231 6 4 28 303 20 240 45 40 29 318 26 229 19 19 30 299 17 247 16 14 31 292 9 257 29 18 32 310 8 255 16 16 33 282 130 252 13 12 34 284 48 233 9 5 35 294 84 258 6 5 36 290 17 237 28 9

[Results of Evaluation]

As shown in Table 1 and Table 2, the fire-resistant steel materials of the invention examples produced by the steel compositions and production conditions defined by the present invention had a 600° C. yield strength of 157 MPa or more in the case of a room temperature tensile strength of 400 to 489 MPa and of 217 MPa or more in the case of a room temperature tensile strength of 490 to 610 MPa. At the same time, in the important feature in the present invention, the 600° C. tensile area reduction value of the weld HAZ, as well, 20% or more is secured. It is learned that high temperature deformation properties of the HAZ are secured.

Furthermore, the fire-resistant steel materials of the invention examples had base material and HAZ Charpy absorption energies at 0° C. of 27J or more, so it was learned that the base material low temperature toughness and joint toughness satisfied the needed performance. From these evaluation results, it is clear that the fire-resistant steel materials of the present invention are superior in high temperature strength and base material and weld joint toughness.

Further, the fire-resistant steel materials of the invention examples all included area fraction 80% or more ferrite phases. Further, the total area fraction of the bainite phase, martensite phase, and MA phase, with the ferrite phase providing the balance, becomes less than 20% in the invention examples. Note that, inclusions were observed in addition to the ferrite phase, bainite phase, martensite phase, and MA phase, but the area fractions were extremely small, so these could be ignored.

As compared with the above fire-resistant steel materials of the invention examples, the steel materials of the comparative examples failed to satisfy either the chemical composition or production conditions defined by the present invention, so either the 600° C. yield strength (600° C. YS), reduction area at the broken part in the HAZ 600° C. tensile test, 0° C. base material Charpy absorption energy, or 0° C. HAZ Charpy absorption energy could not satisfy the targeted characteristic.

From the results of the examples explained above, it is clear that the fire-resistant steel material of the present invention is superior in base material high temperature strength and weld heat affected zone low temperature toughness and reheat embrittlement resistance. 

1. A fire-resistant steel material superior in weld heat affected zone reheat embrittlement resistance and low temperature toughness characterized by containing, by mass %, C: 0.012% to 0.050%, Si: 0.01% to 0.50%, Mn: 0.80% to 2.00%, Cr: 0.80% to 1.90%, Nb: 0.01% to 0.05%, N: 0.001% to 0.006%, Ti: 0.010% to 0.030%, and Al: 0.005% to 0.10%, further limiting the contents of Cu, Mo, B, P, S, and O to: Cu: 0.10% or less, Mo: less than 0.01%, B: less than 0.0003%, P: less than 0.02%, S: less than 0.01%, and less than 0.01%, and having a balance of Fe and unavoidable impurities, having contents of C, Mn, Cr, Nb, and Cu [mass %] satisfying −1200C−20Mn+30Cr−330Nb−120Cu≧−80, having a steel structure as observed by an optical microscope of an area fraction of 80% or more of a ferrite phase, and having a balance of the steel structure of a bainite phase, martensite phase, and mixed martensite-austenite structure.
 2. A fire-resistant steel material superior in weld heat affected zone reheat embrittlement resistance and low temperature toughness as set forth in claim 1 characterized by further containing, by mass %, one or both of V: 0.40% or less and Ni: 1.00% or less.
 3. A fin-resistant steel material superior in weld heat affected zone reheat embrittlement resistance and low temperature toughness as set forth in claim 1 characterized by further containing, by mass %, Zr: 0.010% or less, Mg: 0.005% or less, Ca: 0.005% or less, Y: 0.050% or less, La: 0.050% or less, and Ce: 0.050% or less.
 4. A method of production of a fire-resistant steel material superior in weld heat affected zone reheat embrittlement resistance and low temperature toughness characterized by heating a steel slab having the steel compositions as set forth in claim 1 to 1150 to 1300° C. temperature, then hot working or hot rolling it at 800° C. to 900° C. temperature by a reduction ratio of 50% or more, then allowing it to cool.
 5. A method of production of a fire-resistant steel material superior in weld heat affected zone reheat embrittlement resistance and low temperature toughness characterized by applying the method of production as set forth in claim 4, then treating the steel material at a 400° C. to less than 650° C. temperature range for 5 minutes to 360 minutes for tempering heat treatment. 